1. Field of Invention
The invention relates to a control valve that selects an oil path using a spool that moves according to a signal pressure. The invention also relates to a hydraulic control device for continuously variable transmission devices wherein up-shift, fixed transmission ratio, and down-shift positions are selected based on displacement of the spool of the control valve.
2. Description of Related Art
In V-belt continuously variable transmission devices, a belt is generally disposed between two sets of pulleys having a fixed sheave and a movable sheave. The tension on the belt is changed by moving the movable sheave, thus changing the pulley ratio.
In this type of automatic transmission, the use of a signal pressure from a solenoid valve to select between up-shift, fixed transmission ratio, and down-shift positions, is known (e.g., Japanese laid-open publication number 8-178049). In this technology, the signal pressure from the solenoid valve is varied continuously to move the spool of the transmission control valve. The oil path is selected based on the spool position in order to supply, maintain, and drain the hydraulic pressure to the hydraulic actuator of a movable sheave. Providing the up-shift, fixed transmission ratio, and down-shift positions is performed by the supplying, maintaining, and draining of the hydraulic pressure, respectively.
In the continuously variable transmission described above, the spool position corresponding to the fixed transmission ratio is located between the up-shift and down-shift positions. By providing a large land width on the spool, it is possible to prevent hunting from the fixed transmission ratio position to the up-shift or down-shift position even if there is some spool displacement caused by changes in the solenoid valve signal. However, a larger land width on the spool results in a longer transmission control valve.
Also, although the spool position for fixed transmission ratio can be maintained by using a large land width for the spool, but this results in a decrease in the reliability of the control operation since the spool is always being displaced by changes in the signal pressure.